In Chinese cooking, fish broth is often made from fishes with rather loose flesh which disintegrate easily; this way the fish could fully release its flavor into the broth. A common type of fish is Carassius auratus (which is the same species as the common goldfish, but a different breed which is suitable for cooking).
You cook your fish and the flesh and bones all disintegrate. This gets messy, so the broth is strained and the solid remnants are disposed. Then you add other ingredients to the soup. The moral of the story here is, however, that the soup and the flesh need not come from the same fish!
You may want your stew to contain grouper flesh, but your broth need not be made of grouper. If you like, you can make the broth out of sea bass, perch or carp, and then strain the broth. In fact, this is likely how it works in large Cantonese kitchens: since it would be impossible to prepare fish broth for every single order, there is probably just a large pot of fish broth sitting in the kitchen serving as a base.